United States Homeland security requirements set forth a need for devices capable of sensitive detection of gamma rays originating from hidden radioactive material (e.g. ANSI N42.32, IEC 62401). Conventional technology includes commercial grade radiation detectors that are sensitive enough to easily detect small increases in the intensity of a gamma radiation field, which could be of interest regarding the detection of hidden radioactive sources. In certain cases, such radiation detectors are configured as portable devices that measure a so-called dose rate, which is a measure of the biological impact of the current radiation field. A time integration of the dose rate yields the accumulated dose value, which is of significant interest for the user of the radiation measurement device.
Different dose units are in use in various parts of the world and are preferred by different user groups. For example, dose rate can be measures as absorbed dose rate (Gray/hour); ambient dose equivalent rate (Sv/hour), exposure rate (R/hour). For details regarding various conventional definitions, see the ICRU Report 51 of the international Commission on Radiations. In this document the terms “dose rate” and “dose” shall be understood as a generic term for all these quantities.
In this document the term “gamma” or “gamma radiation” is used for any photon radiation above 5 keV (gamma and X-ray radiation). However, by applying or using a gamma count analysis using the highly sensitive detector, which is required for the detection of very small amounts of radiation, the useful measuring range with respect to the dose rate is very limited. Due to saturation above certain gamma count values, these devices are not able to measure radiation at significantly higher levels that are hazardous to a person exposed to such radiation. In contradistinction to the highly sensitive, low-level radiation detector device as discussed above, other types of commercial radiation devices are able to accurately measure levels of radiation well above a background level. However, such devices typically are not sensitive enough to accurately measure a presence of hidden radioactive material, which is known to be present based on a presence of radiation near or just above a natural background radiation level.
Attempts have been made to combine two different types of radiation detector systems into a single portable radiation-measuring device to detect radiation in a broad dose range such as levels just above a background level as well as levels much higher than a background level. One of two such radiation detector systems in the single portable device can be configured to detect radiation at a range just above natural radiation background level, but cannot be configured to measure the dose rate at higher levels of radiation. Another of the two different radiation detector systems can be configured to measure radiation well above the natural background radiation level, but cannot be configured to detect the lower levels of radiation. These different subsystems can produce very inaccurate and inconsistent measurement results in the respective measuring ranges due to different directional and energy response behavior of the different detectors.